1. Field of Invention
The present invention relates to a delay circuit with constant time delay independent of temperature variations.
2. Description of Related Art
In memory devices, such as DRAM, RC timing tracking is important. Also, RC timing may dominate delay amount of delay circuits and accordingly small RC timing variations are better. In general, the operation temperature ranges between +100° C.˜−40° C. In this operation temperature range, RC timing may have large variations.
FIG. 1 shows a prior art delay circuit. This delay circuit may be used as output circuits. The delay circuit includes several, 5 for example, delay stages 101. The delay stage 101 includes an inverting receiver (formed by PMOS transistors P11, NMOS transistor N11 and resistor R1), a capacitor C1 and an output inverter (formed by PMOS transistors P12 and NMOS transistor N12).
In this prior delay circuit, R1 and C1 are used to compensate temperature variations of the input signal IN for generating an output signal OUT independent of temperature variations.
As known, operating characteristics of transistors vary with temperature changes. Transistors may operate slowly under high temperature and quickly under low temperature on the contrary. As a result, the delay time of the delay circuit undesirably varies with temperature. Changes in the operating temperature of delay circuits may cause its delay time to drift from its specified value.
The resistor R1 may be formed of polysilicon, for example, a polysilicon resistor, and the capacitor C1 may be formed using a gate capacitance of a transistor.
FIG. 2 shows a timing diagram between signals (Q1, O1 and O2) and temperature variations of FIG. 1. Under high temperature, the current of MOS transistor is decreased, so the rising delay amount caused by FIG. 1 is increased. As shown in FIG. 2, temperature varies from +100° C.˜−40° C. and the output signals O1 and O2 are divergent. This means, the output signal OUT is temperature dependent.
Thus, there is a need for an improved output circuit that almost maintains a specified delay time despite temperature variations.